[codex] Ignore defaulted env references in review checks

[joerucci]

Summary

Updates review reference handling so env references with explicit fallback/default arguments are tracked as defaulted instead of required. Defaulted references no longer create blocking missing encrypted value findings, missing schema warnings, or missing .env.example warnings on their own.

Root Cause

The review scanner treated every env(...)-style reference as a required Ghostable-managed value. That made default-backed framework config values, such as Laravel Cashier settings, block review even when the framework provides safe defaults and the value does not need to exist in ENV.

Changes

• Adds default-aware metadata to changed env references.
• Applies missing encrypted value, schema, and .env.example checks only to required references.
• Supports explicit default arguments for PHP/Laravel env, Python os.getenv / os.environ.get / environ.get, and Ruby ENV.fetch.
• Tightens PHP getenv() matching so it does not also match Python os.getenv().
• Adds regression tests covering defaulted Cashier-style config keys and still-required plain env references.

Validation

• go test ./internal/review
• go test ./...